Sunday, September 10, 2017

Clock 3 drive updates

Now that Clock #3's escapement seems to be working, I replaced the center pinion with a v-pulley.  I made the v-pulley from two 1/8" plywood disks in which the edges were beveled in the lathe.  I used hot glue to adhere the disks together, and used a heat gun to ensure the pulley was evenly glued.  The v-pulley was secured to the center wheel by a pin.  It seemed that this would grip an 80 lb monofilament fishing line if a counterweight was used.  The line settles into the groove for an effective radius of 0.75 inches.  The clock ran for over an hour on 3.25 lb drive and 0.5 lb counterweight, which works out to just about 2 inch points of torque on the center wheel. 

I also noticed previously that escape wheel wheel drifted up on its arbor, so I added a small wooden washer cap that press fit to the arbor.  This seemed to work well enough.

Tuesday, September 5, 2017

Clock 3 torsion pendulum updates

I made several updates to Clock #3 over the weekend... in the end, it is running with about 1.75 inch-pounds of torque on the center wheel. 

Paradoxically perhaps, I found that it runs better with the right angle transmission meshing at the top rather than the bottom...

But then I found that the pivot below was unnecessary.  This reduced friction somewhat, and lengthening the pivot considerably was helpful. 

Initially this seemed to reduce the needed torque to around 1 inch pound.  With two pounds on the balance (one pound is shown above), this seemed very stable.  The center pivot was set in the wood frame.  Unfortunately, but since I had to make several drillings to get the depth correct, the hole walls were weak and eventually split.  So I inserted a brass bushing...

This bushing was not depthed correctly, so I had to drill out and shim the hole, so it looks less nice than it does above.  With the bushing, the running torque is back to 1.75 inch pounds...

Additionally, I found that occasionally the escape wheel would skip.  The reason is the when the wheel rides up on the locking detent (black arrow), it deflects the detent too far and the wheel slips past...

Sunday, August 27, 2017

Clock 3 torsion pendulum Q

So the pivots on Clock #3 clearly are consuming energy... I tried the torsion pendulum outside the frame (basically just excluding the pivot).  I counted 70 periods before the amplitude halved... which yields a Q of roughly 315.  But there is considerable wobble since the balance itself is badly out of poise.  I tried to clean this up a bit on the lathe and redrill the center hole (which is misaligned).  Repositioning the suspension hanger also helped, so now the rod spins vertically without much wobble except when the impulse is given.

This seemed to help a bit, as the necessary drive torque has dropped to 1.75 inch-pounds, or about 0.34 mW.  This is about 2/3 as much power as Clock #1 uses, which gives basically one day of runtime with about the biggest weight I'm comfortable with.

Much better, but I suspect that there is still easily fixable power loss due to the wobble at impulse.  If the pendulum weight is placed lower (on a longer rod), this should be reduced, and might help matters further.

Wooden clock power consumption

For reference...

Clock #1 is powered by a 10 lb drive weight that falls 46" in 27 hours.
Power = (10 lb * 4.448222 N/lb)*(46 in*0.0254 m/in)/(27 h * 3600 s/h) = 0.5 mW

Clock #2 is powered by a 1 lb 4 oz weight that falls 11.5" in 5.5 minutes.
Power = (1.25 lb * 4.448222 N/lb)*(11.5 in*0.0254 m/in)/(5.5 min * 60 s/min) = 5 mW

Clock #3 (as it currently stands) is powered by a 4.125 inch-pound torque on the center wheel.
Power = (4.125 lb*4.448222 N/lb)*(6.28319 in/rotation * 0.0254 m/in)/3600 s/rotation = 0.8 mW

This explains why Clock #3 power consumption seems high... Torsion balances are supposed to be much more efficient (if lower Q) than pendulums, but this one is not!

Saturday, August 26, 2017

Clock #3 at least runs!

Clock #3 has been plagued by various problems, mostly with the resonator/balance not being really suitable.  One of the things that was exacerbating matters was the passing spring on the detent was still (even after modifications) too stiff.  I went back to various books and found that the passing spring tended to be quite long, passing beyond the locking pallet.  So I modified the spring in that way.  Now it's much lighter.

I thought long and hard about the balance issues, and came to the conclusion that I probably had to face a right angle turn and a torsion balance.  You can see the new right-angle transmission wheel above on the impulse roller assembly.  The other half mounts to the balance through a spring, which consists of a spring, a rod, pin, wheel, and balance (at the bottom of the rod).  Here are the pieces...

... and here it is assembled.

The balance hangs in front of the lower section of the clock frame.  The rod passes through a pivot, which is formed from a steel wire.

This is perhaps not the best option, but it has the merit of making it easy to adjust the depth of the right-angle transmission.  The transmission runs fairly smoothly and noiselessly.

To first approximation, the new balance appears to have a resonant Q of around 12, whether loaded or not.  (Eyeballing 3 periods before the balance amplitude halves, then multiply by 4 according the rule of thumb suggested by Woodward.)  So the pivot and transmission is the least of my concerns, but the fact that the unloaded Q is low is definitely an issue.

The mechanism appears to run semi-reliably...

but it needs considerably more torque than I'd like.  The center wheel needs about 4.125 inch-pounds in order to maintain balance amplitude.  This is concerning because I was really hoping this to be the torque on the drive wheel -- a factor of 10 weaker!  The resonator having so much absolute loss of power is clearly a problem.

Lower on the priority list is the fact that with this balance the clock runs a bit fast.   It makes 17 "loud" ticks per minute rather than 15.  That should be easily corrected.

Thursday, August 17, 2017

Clock 1 humidity redux

I think I have finally figured out what goes wrong with Clock 1 when the humidity changes.  After much watching and poking, I found that the third wheel appeared to be sticking slightly, at least whenever the clock stopped.  Carefully pulling the third wheel backwards once the clock stopped, I could feel some rubbing...

I think the third wheel pinion expanded a bit beyond where it was supposed to, or maybe the frame expanded, but in any event, it was fouling at the tip.  After ensuring that the pivot was rigidly planted where it was supposed to be, I took to very slightly filing the addenda of the pinion.

This makes the pinions closer to an ogive form.  I don't think this really matters, because the clock is now happily running even though the humidity has been around 60%.  Hopefully I won't run into an "opposite" problem when the humidity drops later in the year.

Monday, May 29, 2017

Clock #1 summertime adjustments...

Clock #1 has been in our dining room since I moved it upstairs since March 11.  It ran for several months, and then in mid-April started to become unreliable.  I suspected weather factors were the cause.  After running from mid-April until mid-May, it stopped again, this time apparently for good.  After letting it sit for about 10 days, I went back to investigate the issue.

The overall friction appears to be higher in the spring/summer (though I don't know for certain), although it's unclear exactly if this is localized to a specific mesh in the train..

It seems that the problem was that a few escape teeth were too long by a very small amount (8/1000").  When the friction is lower in the train, the pendulum amplitude is high enough that this doesn't matter.  But when the amplitude drops, this becomes a problem.

To debug the problem, I stuck a post-it note to the back of the frame behind the pendulum.  I marked on this paper the precise pendulum location when each escape tooth released.   The marks were about 13" from the pendulum pivot, and indicate that the typical distance between entry and exit release was 3/16", or about 0.8 degrees peak-to-peak.  So if the pendulum swings less than that, the clock is likely to stop.  I found that one tooth that seemed to cause stoppage consistently corresponded to a mark 1/16" farther out from the rest.  This means that an additional 0.15 degrees was necessary to escape that tooth.  Since the anchor has a length of 1 3/4", this translates to an escape tooth of about (1/16)*(7/4)/13 = 8/1000" longer than the rest.  That's a very small amount, but easily corrected with a file.  Once I corrected that, I checked each other tooth as well, adjusting them so they were all within the 0.4 degree peak-to-peak release margin. 

It seems unlikely that expansion or contraction of the escapement itself (due to weather) is the cause of the clock's malaise, especially because that particular tooth was already marked as being problematic in the past!  Hopefully, my guess about balance amplitude is correct, because it seems to also explain the other issues about the escape wheel sitting at the front/back of the clock too.  This slightly shifts the escape wheel up/down by a very small amount and seems to change the effective length of the escape wheel teeth by a few thousandths of an inch.